The present invention is directed to pulsed CO.sub.2 lasers, and more particularly to a CO.sub.2 laser that eliminates the nitrogen tail from the output pulse of the laser.
The pulsed CO.sub.2 laser is a source of laser energy in the 9-11 micron spectral regime. For, most effective performance, the laser operates with an active gaseous mixture consisting predominantly of helium, nitrogen, and CO.sub.2. The presence of nitrogen substantially improves the energy output of the laser. CO.sub.2 laser pulses utilizing nitrogen in the discharge invariably exhibit a short gain switched spike on the order of one hundred nanoseconds, followed by a long tail on the order of one microsecond resulting from the nitrogen in the mixture. Typically, approximately one half the total pulse energy is contained in this so called "nitrogen tail". If one attempts to remove the nitrogen from the laser gas mix to eliminate the tail (by pumping a CO.sub.2 - He mix, for example) the tail disappears but the output energy is reduced by even more than the factor of two because of the degradation of the initial electron distribution that necessarily ensues. Thus, removal of the nitrogen to remove the tail results in substantially degraded laser performance. For applications such as harmonic generation in a nonlinear material where the instantaneous power conversion is a function of the intensity of the laser pulse squared, the nitrogen tail on the laser pulse is of considerable detriment. The "nitrogen tail" introduces additional energy loading on the material which, because of its relatively low intensity, contributes little to the nonlinear energy conversion process. The additional energy loading is undesirable because it leads to additional heating of the nonlinear material and/or its surface or surface coatings and consequently material damage, without contributing in any substantial way to the nonlinear conversion process.
Heretofore, the only means of reducing the nitrogen tail besides the elimination of nitrogen from the laser gas mix as already described, was use optical switch techniques external to the laser cavity to essentially chop off the of the laser pulse. By the use of well-developed electrooptic switching technology such pulse truncation is possible. However, this approach suffers from several drawbacks. Approximately one-half the laser pulse energy is thrown away and the switch is generally made of either CdTe or GaAs which are both difficult crystals to grow. These crystals are also expensive and are limited in available size to roughly one centimeter by one centimeter aperture. In addition, the use of such an optical switch, along with the external electrical switch (spark gap) and associated electronics need, introduces its own damage problems and limitations, so that the damage problem of the nonlinear frequency converter material is transferred more or less to a damage problem of the optical switch itself.